An optical memory technology which uses optical recording media having pit-shaped patterns as high-density and high-capacity recording media is increasing its applications to wider ranges such as digital audio discs, video discs, text file discs, and also data files. The optical memory technology records and reproduces information on optical recording media such as optical discs via finely condensed optical beams.
Its fundamental recording mechanism is briefly described as follows. For example, in the case of recording information onto an optical disc made of phase changing material, light with power relatively higher than the light irradiated in the case of reproducing information is irradiated onto the optical disc. The light irradiation induces a phase change in the material of the recording surface to create areas having different indexes of reflection, with the result that information is recorded or erased. Further, at the time of reproduction, light in an amount small enough not to induce the phase change is irradiated onto the optical disc. The reproduction of information is performed by detecting the change in the index of reflection of the irradiated light.
The recording and reproduction operations described above exclusively depend on their optical system. Basic functions of the optical head device, which is a main constituent element of the optical system, are roughly categorized into a converging function of forming minute spots of a diffraction limit by use of the light emitted from the light source, a focus control and tracking control function for the optical system, and a function of detecting pit signals. These functions are realized, in accordance with their individual purposes and applications, in combination of various optical systems and photoelectric conversion detecting methods.
One element constituting the basis of the optical system is a light source. In general, a semiconductor laser is preferable as a light source for collecting light up to its diffraction limit. In an optical head device, a small-sized semiconductor laser is mainly used as a light source. In order to perform recording and reproduction with high reliability, the semiconductor laser to be used as a light source of the optical head is naturally required to have low noises.
Semiconductor lasers are roughly categorized into two types: a single-mode laser and a multi-mode laser. Among them, the single-mode laser has a problem that the wavelength of its emitting light is discretely changed due to the influence of the light returned from the optical disc and the like (referred to as a mode hop), and the change in the light amount accompanying the wavelength change is contained in the recording and reproducing signal as a noise. If the semiconductor laser is of a type that is largely influenced by the returned light, a large influence also appears on its laser oscillation itself. In this case, its oscillation may be unstable and its output may largely vary. In this state as it is, the recording and reproduction are also unstable, resulting in poor signal quality.
On the other hand, the multi-mode laser emits light with plural wavelengths from the beginning, and is little influenced by noises caused by a mode hop, and therefore, is excellent as a light source for use in an optical head. However, it is difficult to constitute a multi-mode laser for some desired wavelengths and there are some cases where a desired wavelength is obtainable only in a single-mode laser. Further, depending on environmental conditions such as high temperature, the operation of the multi-mode laser may be unstable and its operation mode may change into a single mode.
In an attempt to solve such a problem, a method in which high frequency superimposing is applied to a single-mode laser to change it into a multi-mode laser and the thus-formed laser is used as a multi-mode laser is employed. Specifically, alternating current components at several hundreds MHz frequency obtained from an oscillating circuit of a high-frequency superimposed circuit are superimposed onto a laser driving current to allow the laser to operate in a multi-mode. In this manner, a practical light source affected by a suppressed level of the returned light and having a low noise is realized.
FIG. 19 is a block diagram showing a structure of a conventional semiconductor laser driving device constituted as described above. A semiconductor laser driving device 150 includes a semiconductor laser 61, a photodetecting element 62, a high-frequency superimposing circuit 72, a laser driving circuit 64, and a high-frequency superimposing control circuit 65. Further, the high-frequency superimposing circuit 72 includes an oscillating circuit 63, a driving power source 66, and a capacity element 70. The laser driving circuit 64 supplies a driving current Id to the semiconductor laser 61. The semiconductor laser 61 emits light when the driving current Id flows into it. The semiconductor laser 61 is a single-mode laser. The photodetecting element 62 receives a part of the light emitted from the semiconductor laser 61 and performs photoelectrical conversion to the received light, thus outputs an electric signal Vs which is a light intensity detecting signal proportional to the light amount (light intensity). The laser driving circuit 64 monitors the electric signal Vs supplied from the photodetecting element 62, and controls the driving current Id in such a manner that the electric signal Vs takes a constant value. By the employment of the structure described above, the semiconductor laser driving device 150 can allow the semiconductor laser 61 to emit light at a desired output level.
The high-frequency superimposing circuit 72 is a circuit for superimposing a high-frequency signal Uf onto the driving current Id. The oscillating circuit 63 oscillates by receiving the supply of electric power from the driving power source 66. The high-frequency signal Uf that the oscillating circuit 63 outputs is transmitted to the path of the driving current If via the capacity element 70 that cuts off the direct current components. At this time, by properly setting the oscillating amplitude and frequency of the oscillating circuit 63, the semiconductor laser 61 is enabled to operate as a multi-mode laser. As a result of this, the noise of the semiconductor laser 61 caused by the returned light can be suppressed, and stable reproduction of the information from the optical disc can be performed.
At this time, the change in the light emitted from the semiconductor laser 61 with respect to time is represented by the solid curve 51 in FIG. 20, for example. As is exemplarily illustrated in FIG. 20, the intensity of the emitted light contains alternating current components having a frequency that corresponds to the frequency of the high-frequency signal Uf outputted by the high-frequency superimposing circuit 72 due to the influence of the high-frequency superimposing. However, if the frequency of the high-frequency signal Uf is set to a value sufficiently higher than the frequency band of the reproducing signal of the optical recording medium, by properly selecting the frequency characteristics of the photodetector that detects the reproducing signal, it is possible to obtain a signal that is the same signal as of the case where reproduction is performed by the laser beam having only the direct current components of the magnitude same as an average value in terms of time shown by a wave line 53. The photodetecting element 62, due to its frequency characteristics, outputs a time average value shown by the broken line 53 as the electric signal Vs.
However, in the structure described above, the peak value 52 of the light amount is higher than the average value of the light amount. For this reason, the power of the laser beam is larger than the average value during a very short period of time. Therefore, if the reproduction of information is performed by irradiating the light emitted from the semiconductor laser 61 onto the optical disc, although the detected reproducing signal is the same as the signal detected by the emitted light having a power of the average value, there arise some cases where the optical disc may cause phase change although it is a small change. This is equivalent to overwrite or erase information during reproducing the information even if the overwritten or erased portion is slight. As a result, the original information recorded on the optical disc deteriorates.
Japanese Unexamined Patent Publication No. 2001-352124 discloses, in order to solve the problem that the frequency of the high-frequency signal varies as the element constant of the circuit element of the semiconductor laser driving device varies with the change in the temperature, a technology that has enabled variable control of the frequency. However, although the conventional technology disclosed in this Reference overcomes the deviation in the frequency of the high-frequency signal, it does not overcome the problem of the deterioration in the information caused by the above-described peak power of the emitted light.
As described above, the conventional semiconductor laser driving device has a problem that there may be a case where deterioration in the information recorded on the optical recording medium is introduced by the peak power of the emitted light.